The present invention relates to a system for detecting the air-fuel ratio, or more in particular to an air-fuel ratio detection system capable of using a system for controlling the air-fuel ratio of an internal combustion engine.
A conventional air-fuel ratio sensor, as disclosed in U.S. Pat. Nos. 3,691,023 and 4,158,166, comprises a solid electrolyte having a diffusion resistor and operates in such a manner that the oxygen is drawn out of the diffusion resistor and a measurement is taken of a current value as of the time when the concentration of the oxygen is reduced to zero (what is called the critical current value). In this method, the measurement of the air excess rate .lambda. is possible only in the region where the oxygen is contained in the gas to be measured, that is, in the region where the air excess rate is 1.0 or more, while if the value .lambda. is less than 1.0 it is impossible to measure the value .lambda..
Also when measuring the region where the value .lambda. is smaller than 1.0, it is necessary to reverse the voltage applied to the solid electrolyte at the point 1.0 in the value .lambda. as indicated in Japanese Patent Unexamined Publication No. 166039/80. This requires a new means for detecting the point where .lambda. is 1.0, complicating the system.
According to the method disclosed in SAE paper 810433 shown in FIG. 12, on the other hand, the current is applied to one solid electrolyte while being reversed in polarity, and by loading and unloading the oxygen in a chamber, the oxygen concentration in the chamber is changed, so that the change in electromotive force with the concentration is detected by the other solid electrolyte. The polarity is reversed each time the electromotive force reaches a predetermined value, and the fact is taken advantage of that the period of this reversal is proportional to the oxygen concentration. In another method disclosed in U.S. Pat. No. 3,907,657, the oxygen diffused in a chamber is drawn out by a current, and the time .tau. before the oxygen concentration in the chamber is reduced to zero is counted from the change in electromotive force between the electrodes on the solid electrolyte, utilizing the phenomenon that the value .tau. is proportional to the oxygen concentration The absolute amount of oxygen in a chamber of a predetermined volume is measured in both the methods, which are liable to be affected by the secular variations in the volume and temperature or pressure. Further, the predetermined volume is formed by a single aperture, and the effect is great if the aperture is clogged. Still another disadvantages of these systems is that since the whole sensor is exposed to the combusion exhaust gas, it is impossible to measure the value .lambda. in the region where it is smaller than 1.0.
A further conventional system is disclosed in Japanese Patent Unexamined Publication No. 192852/82, in which a means for detecting the temperature of a solid electrolyte comprises an alternating current with the solid electrolyte, a period of temperature measurement, or a temperature-measuring device. A method is also known in which the temperature of the electrolyte is estimated from the operating conditions of an engine or the temperature of the exhaust gas thereof (Japanese Utility Model Unexamined Publication No. 103265/84, Japanese Patent Unexamined Publication No. 188054/84). These systems, however, have a complicated construction or an insufficient accuracy of temperature detection which is effected indirectly.